Receivers for digital communications systems are becoming available which can handle several transmissions simultaneously. Such receivers typically make use of multiuser detection, commonly referred to as MUD. Multiuser Detection is a means by which several signals either completely or partially occupying a single communications channel can be separated mathematically.
To operate, a MUD receiver must have the received data available, and must have knowledge of the basic waveform transmitted at each transmitter, as appearing in the receiver. This basic waveform is commonly referred to as a composite signature waveform. Each transmission's composite signature waveform is the waveform that would be present in the receiver, if only one data symbol had been transmitted by each transmitter individually. These waveforms each define a column in an ‘S’ matrix in the MUD receiver, sometimes called the signature matrix. To compute these waveforms, and the signature matrix, some means of parameter estimation is necessary.
The parameter estimates for a MUD receiver can be calculated once for every frame of data being transmitted, and the quality of these estimates varies over the length of the associated frame. This situation becomes especially problematic as the frame length is increased. In particular, mismatches between the transmitter and receiver carrier frequencies, which is common in all communications systems, cause the received signals to drift in phase rotation as the frame progresses. This mismatch is most typically due to accuracy limitations of the local oscillators in both the transmitters and receivers, and can also be caused by Doppler shifts due to moving transmitters and/or receivers. If the frequency mismatch is not accurately corrected for, each user's symbol sequence estimates at the output of the MUD module will exhibit a unique constant phase drift and offset. Thus, frequency compensation is necessary.
In a single user communication system, the estimation of the carrier frequency difference between the transmitter and the receiver is performed before the symbol estimates are made. This carrier mismatch is usually estimated and compensated for using a phased lock loop at the front end of the receiver. This same technique is not possible in a multiuser detection system because the received signal is composed of the superposition of each transmission signal as seen at the receiver, and the frequency mismatch is unique between the receiver and each transmitter. Therefore, a technique is needed to estimate and correct for the effects of the frequency mismatch that is unique between the receiver and each transmitter.
In a typical MUD communication system, bursts of training data are repeatedly sent during a transmission, and parameter estimates are made at the receiver based on these training bursts. Making more frequent parameter estimates at the receiver could help compensate for carrier mismatch. This could be accomplished by decreasing the frame length, or making more frequent bursts of training data. This compensation technique for handling carrier mismatch drift, however, is associated with a number of disadvantages.
One of the more significant disadvantages is that the overhead of the system is increased. This means that for a fixed signaling rate of the system, training data will replace the payload data, thereby decreasing throughput of payload data or decreasing the overall information carrying capacity of the channel. If more sophisticated estimators are attempted, the frequency of training-estimate passes may be held more or less constant, but these methods typically imply more training support (e.g., bigger training packets). This increase in training support has a similar effect in that payload data is being displaced by training data and overall capacity is decreased.
What is needed, therefore, are techniques for performing frequency mismatch compensation for multiuser detection applications.